4. SIDEDNESS AND RELATIVISTIC FLOWS ON LARGE SCALES

The aim of this section is to discuss recent evidence bearing on the
question "why
are strong-flavor jets predominantly one-sided?". Three main classes of
explanation have been proposed:

Doppler boosting: Jets are intrinsically symmetrical and have
relativistic bulk
velocities. The nearer jet appears to be brighter as a result of Doppler
boosting. Most
attention has been focused on this idea, since it makes several simple
predictions.
With the exception of M 87 (Biretta, these proceedings), there is no
direct evidence
for apparent motions in excess of c on kiloparsec scales. The indirect
arguments are:

Continuity from parsec scales There is good evidence for
relativistic flow in
VLBI jets (e.g. Marscher, these proceedings) and the continuity
of sidedness
from parsec to kiloparsec scales implies that the mechanism for generating the
asymmetry should be independent of scale.

Depolarization asymmetryLaing (1988) and
Garrington et al. (1988,
1991)
showed that, in FRII radio sources with one-sided jets (predominantly quasars),
the jetted lobe depolarizes less rapidly with increasing wavelength than the
unjetted lobe. The obvious explanation
(Laing 1988;
Garrington & Conway
1991)
is that the nearer lobe is seen through less magnetoionic material (hot gas
in the galaxy or cluster halo, primarily) and therefore shows less
depolarization.
This effect is discussed in more detail in the following subsection.

Emission-line velocitiesvan Breugel (1989)
noted a tendency for the stronger (or
only) jet to be on the side of the source with blueshifted
forbidden-line emission
and the counter-jet side to have redshifted lines. It is extremely important to
confirm this result, as it is the only direct piece of evidence
for motion towards
the observer associated with the stronger jet (the velocities are, of
course, much
less than those inferred for the jet and are presumed to be due to interaction
with outflowing material).

Asymmetric dissipation: One of the jets radiates a significantly
higher proportion
of its energy as synchrotron radiation. Models for such a process are not yet
well-developed (but see
Fraix-Burnet 1992
for a possible approach).

Alternating ejection: Only one jet is active at any time. This
idea encounters severe
difficulties when applied to objects where the synchrotron lifetimes of
electrons
in components on both sides of the nucleus are short compared with the
light-travel
times between components and nucleus. This was originally pointed out by
Hargrave & Ryle (1974)
for Cygnus A and applies to many other powerful radio
sources. The recent detection of optical synchrotron radiation from the
counterjet side of M 87
(Stiavelli et al. 1992;
Sparks et al. 1992)
allows a similar argument to be made for an FRI source.

For the moment, we adopt the hypothesis that strong-flavor jets are
symmetrical and relativistic and attempt to test its predictions.